The ring laser gyroscope was developed as a logical replacement for the early mechanical inertial gyroscopes since ring laser gyroscopes have minimal moving parts thereby allowing extremely accurate rotational sensing. Based upon the principles of the Sagnac Effect, a ring laser gyroscope has at least two counter-propagating electromagnetic waves, typically light waves, that oscillate within an optical ring cavity. When an ideal ring laser gyroscope is stationary, no rotation is indicated by the sensor. However, when the ring cavity of the laser gyroscope is rotated about its central axis, the counter-propagating electromagnetic waves develop a beat frequency. Well above a characteristic mode locking zone, a linear relationship between this beat frequency and the rotation rate of the gyroscope with respect to an inertial frame of reference may be established.
Typically, a working ring laser gyroscope requires adjusting so as to approach the ideal linear relationship between the beat frequency and the rotation rate of the gyroscope. Such adjusting includes rate biasing or mechanical dithering to prevent the counter-propagating waves from mode locking at low rotation rates. This mode locking phenomenon, known as lock-in, occurs when the counter-propagating waves couple to the same frequency. If the rotation rate of a ring laser gyroscope starts at a value above that of where lock-in occurs, and is then decreased, the frequency difference between the two counter-propagating waves disappears at a certain rotation. Thus, lock-in always occurs at low rotation rates and results in the loss of rotational rate information.
Various techniques have been developed for adjusting ring laser gyroscopes so as to reduce or eliminate the effects of lock-in. One such technique is disclosed in U.S. Pat. No. 3,851,973, awarded to Macek. In this technique, a magnetic bias mirror is used to impart a non-reciprocal phase shift to two counter-propagating light beams so as to avoid lock-in. The bias mirror includes thin magnetic and dielectric layers disposed on a substrate. The magnetization of the magnetic layer is aligned parallel to the major surface of the mirror and normal to the plane of the ring so as to interact with plane polarized light aligned parallel to the ring plane to produce a non-reciprocal phase shift in the counter-propagating beams without distorting the polarization from the ring plane or converting it to elliptical form. The interaction between the light beams and magnetic field relied on to produce the bias is the transverse Kerr magneto-optic effect.
A major problem with the magnetic bias mirror described above is an undesired non-reciprocal loss or differential reflection of the counter-propagating beams in addition to the desired non-reciprocal or differential phase shift imparted to the beams. The non-reciprocal loss occurs as a consequence of the oppositely directed oscillatory beams being differentially reflected from the bias mirror and is believed to be attributed to the presence in the mirror of the magnetic layer which is characterized by a refractive index having both real and imaginary parts. This non-reciprocal loss of differential reflectivity is deleterious to the ring operation because it is likely to result in an undesired varying internal bias in the presence of backscatter. Furthermore, in the transverse Kerr effect, only p-polarized beams show a magneto-optic effect, and hence a differential phase shift. This means that additional optical elements such as Brewster windows must be introduced into the optical cavity to assure operation only in the p-polarized mode.
Another technique for adjusting ring laser gyroscopes so as to reduce or eliminate the effects of lock-in is disclosed in U.S. Pat. No. 4,410,276, awarded to Ljung et al. In this technique, at least two mirrors in a ring laser gyroscope are vibrated in a direction perpendicular to their reflective surfaces in equal and opposite amounts so as to maintain the total path length traveled by a pair of counter-propagating light beams, thereby resulting in a phase modulation that reduces or eliminates lock-in between the two counter-propagating light beams. This technique follows a simple methodology for phase modulating the two counter-propagating light beams without introducing the undesirable non-reciprocal losses or differential reflections that are present with the above described magnetic bias mirror technique. However, this technique also requires high maintenance mechanical components to control the vibrational movement of the mirrors in a precise manner so as to produce phase modulation in the ring laser gyroscope.
Accordingly, it would be desirable to incorporate the simple methodology proposed in the patent awarded to Ljung et al. for reducing or eliminating the effects of lock-in, while eliminating the need for any mechanical components to control the movement of the mirrors.